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The present invention relates to systems and processes for vaporizing liquefied natural gas. More particularly, the present invention relates to processes and systems whereby liquefied natural gas is vaporized by heat exchange action imparted onto a circulating fluid by heated water. More particularly, the present invention relates to a process and system for the vaporizing of liquefied natural gas where the heated water is elevated in temperature by the blower action of a water tower.
Natural gas often is available in areas remote from where it ultimately will be used. Often, shipment of such natural gas involves marine transportation which makes it desirable to bulk transfer the natural gas by liquefying the natural gas so as to greatly reduce its volume for transportation at essentially atmospheric pressure. Under these conditions, the liquefied natural gas is at a temperature of approximately xe2x88x92162xc2x0 C., though heavier hydrocarbons (such as, for example, ethane, propane, butane, and the like) often vary the boiling point of the liquefied natural gas slightly. Heretofore, a wide variety of heat transfer fluids, systems, and processes, have been proposed for the regasification or vaporization of liquefied natural gas.
In many circumstances, hot water or steam is used to heat the liquefied gas for vaporization. Unfortunately, such hot water or steam often freezes so as to give rise to the hazard of clogging up the evaporator. Various improvements in this process have heretofore been made. The evaporators presently used are mainly of the open rack type, intermediate fluid type and submerged combustion type.
Open rack-type evaporators use sea water as a heat source for countercurrent heat exchange with liquefied natural gas. Evaporators of this type are free of clogging due to freezing, easy to operate and to maintain and are accordingly widely used. However, they inevitably involve icing up on the surface of the lower portion of the heat transfer tube. This consequently produces increased resistance to heat transfer so that the evaporator must be designed to have an increased transfer area, which entails a higher equipment cost. To ensure improved heat efficiency, evaporators of this type include an aluminum alloy heat transfer tube of a special configuration. These types of evaporators are economically disadvantageous.
Instead of vaporizing liquefied natural gas by direct heating with water or steam, evaporators of the intermediate fluid type use propane, fluorinated hydrocarbons or like refrigerant having a low freezing point. The refrigerant is heated with hot water or steam first to utilize the evaporation and condensation of the refrigerant for the vaporization of liquefied natural gas. Evaporators of this type are less expensive to build than those of the open rack-type but require heating means, such as a burner, for the preparation of hot water or steam and are therefore costly to operate due to fuel consumption.
Evaporators of the submerged combustion type comprise a tube immersed in water which is heated with a combustion gas injected thereinto from a burner. Like the intermediate fluid type, the evaporators of the submerged combustion type involve a fuel cost and are expensive to operate.
In the past, various patents have issued for processes and apparatus for the vaporization of liquefied natural gas. For example, U.S. Pat. No. 4,170,115, issued on Oct. 9, 1979 to Ooka et al., describes an apparatus for vaporizing liquefied natural gas using estuarine water. This system is arranged in a series of heat exchangers of the indirect heating, intermediate fluid type. A multitubular concurrent heat exchanger is also utilized in conjunction with a multitubular countercurrent heat exchanger. As a result, salt water is used for the vaporization process. U.S. Pat. No. 4,224,802, issued on Sep. 30, 1980 to the same inventor, describes a variation on this type and also uses estuarine water in a multitubular heat exchanger.
U.S. Pat. No. 4,331,129, issued on May 25, 1982 to Hong et al., teaches the utilization of solar energy for LNG vaporization. The solar energy is used for heating a second fluid, such as water. This second fluid is passed into heat exchange relationship with the liquefied natural gas. The water contains a anti-freeze additive so as to prevent freezing of the water during the vaporization process.
U.S. Pat. No. 4,399,660, issued on Aug. 23, 1983 to Vogler, Jr. et al., describes an atmospheric vaporizer suitable for vaporizing cryogenic liquids on a continuous basis. This device employs heat absorbed from the ambient air. At least three substantially vertical passes are piped together. Each pass includes a center tube with a plurality of fins substantially equally spaced around the tube.
U.S. Pat. No. 5,251,452, issued on Oct. 12, 1993 to L. Z. Widder, also discloses an ambient air vaporizer and heater for cryogenic liquids. This apparatus utilizes a plurality of vertically mounted and parallelly connected heat exchange tubes. Each tube has a plurality of external fins and a plurality of internal peripheral passageways symmetrically arranged in fluid communication with a central opening. A solid bar extends within the central opening for a predetermined length of each tube to increase the rate of heat transfer between the cryogenic fluid in its vapor phase and the ambient air. The fluid is raised from its boiling point at the bottom of the tubes to a temperature at the top suitable for manufacturing and other operations.
U.S. Pat. No. 5,819,542, issued on Oct. 13, 1998 to Christiansen et al., teaches a heat exchange device having a first heat exchanger for evaporation of LNG and a second heat exchanger for superheating of gaseous natural gas. The heat exchangers are arranged for heating these fluids by means of a heating medium and having an outlet which is connected to a mixing device for mixing the heated fluids with the corresponding unheated fluids. The heat exchangers comprise a common housing in which they are provided with separate passages for the fluids. The mixing device, constitutes a unit together with the housing and has a single mixing chamber with one single fluid outlet. In separate passages, there are provided valves for the supply of LNG in the housing and in the mixing chamber.
It is an object of the present invention to provide a process and system whereby liquefied natural gas can be vaporized at minimal cost.
It is another object of the present invention to provide a process and apparatus whereby ambient air can be utilized to provide the heat for the LNG vaporization process.
It is still another object of the present invention to provide a system and process to provide a heat exchange process for the vaporization of liquefied natural gas which is relatively inexpensive, easy to implement and easy to use.
These and other objects and advantages of the present invention will become apparent from a reading of the attached specification and appended claims.
The present invention is a process for vaporizing liquefied natural gas comprising the steps of: (1) passing water into a water tower so as to elevate the temperature of the water; (2) pumping the elevated temperature water through a first heat exchanger; (3) passing a circulating fluid through the first heat exchanger so as to transfer heat from the elevated temperature water into the circulating fluid; (4) passing the liquefied natural gas into a second heat exchanger; (5) pumping the heated circulating fluid from the first heat exchanger into the second heat exchanger so as to transfer heat from the circulating fluid to the liquefied natural gas; and (6) discharging vaporized natural gas from the second heat exchanger.
In the process of the present invention, the step of passing water comprises distributing the water over an interior surface of the water tower and drawing ambient air through the water tower across the distributed water so as to transfer heat from the ambient air into the water. In the preferred embodiment of the present invention, the ambient air will have dry bulb air temperature in excess of 73xc2x0 F. The moisture from the air is condensed within the water tower and this condensed moisture is then drained from the water tower. The cooled air is exhausted from a top of the water tower after the ambient air is drawn across the distributed water. The water tower is formed with a plurality of baffles therein. A blower is positioned at a top of the water tower. The water tower is a plurality of openings formed in a wall thereof adjacent the respective plurality of baffles. The step of drawing in ambient air comprises passing the ambient air through the plurality of openings so as to be in close proximity to the water distributed over the plurality of baffles. A water basin is secured to the bottom of the water tower. This water basin is positioned to collect the heated distributed water. The heated distributed water from the water basin is pumped to the first heat exchanger.
In the method of the present invention, water from the first heat exchanger is pumped to the water tower after the heat is transferred into the circulating fluid.
The second heat exchanger is a shell-and-tubes heat exchanger. The heated circulating fluid is passed within the shell and arround the tubes of the second heat exchanger. The liquefied natural gas passes through the tubes in the second heat exchanger. The circulating fluid from the second heat exchanger is pumped to the first heat exchanger after the heat is transferred from the circulating fluid into the liquefied natural gas.
In the method of the present invention, an auxiliary source for heating the circulating fluid is provided in those circumstances where the ambient temperature of the air is less than 73xc2x0 F. In particular, another quantity of circulating fluid is heated by a heating source other than the water tower. This heated circulating fluid is then passed into the second heat exchanger. In the preferred embodiment of present invention, the secondary heating source is a gas-fired boiler. A small portion of the discharged natural gas must be passed to the boiler so as to be fired for the heating of the circulating fluid.
The present invention is also a system for the vaporizing of liquefied natural gas comprising a water tower means having a water inlet line and a water outlet line, a first heat exchange means connected to the water outlet line such that the heated water passes therethrough, and a second heat exchange means having a liquefied natural gas therein. The water tower means serves to heat the water passed from the water inlet line therein such that the heated water passes to the water outlet line. The first heat exchange means has a circulating fluid line extending therein in heat exchange relationship with the water outlet line. The first heat exchange means serves to transfer heat from the heated water in the water outlet line into the circulating fluid in the circulating fluid line. The circulating fluid line extends in the second heat exchange means in heat exchange relationship with the liquefied natural gas line. The second heat exchange means serves to transfer heat from the heated circulating fluid into the liquefied natural gas in the liquefied natural gas line. The second heat exchange means has a vaporized gas outlet extending therefrom.